© a+ computer science - . public class compsci { } all java programs start with a class


public class CompSci{

}

All Java programs start with a class.


public class CompSci{ public static void main(String[] args) { System.out.println("Comp Sci!"); }} OUTPUT

Comp Sci!


public class CompSci{ //open brace

public static void main(String[] args) { System.out.println("Comp Sci!"); }} //close brace

Braces – You gotta have ‘em! Every classand every method must have a { and a } .


public class CompSci{ public static void main(String[] args) { System.out.println("Comp Sci!"); }}You must put a semi-colon at the end of all Java program statements ( ; ).


Never put a ; before an open { brace

;{ //illegal}; //legal


public class CompSci{ public static void main(String[] args) { System.out.println("Comp Sci!"); }}

Indent all code 3 spaces to make it easier to read.


System.outfrequently used methods

Name Use

print(x) print x and stay on the current line

println(x) print x and move to next line down

printf(s,x) print x according to s specifications


System.out.print("compsci");

reference command / method

OUTPUTcompsci


System.out.print("compsci");System.out.print("compsci");

OUTPUTcompscicompsci


System.out.println("compsci");

OUTPUTcompsci


System.out.println("compsci");System.out.println("compsci");

OUTPUTcompscicompsci


System.out.println("c\tompsci");

\n newline\t tab\r carriage return\b backspace

OUTPUTc ompsci


System.out.println("com\tpsci");

OUTPUTcom psci



System.out.println("comp\nsci");

OUTPUTcompsci



\\ outs \\" outs "\’ outs ’

System.out.println("\\compsci\"/");

OUTPUT\compsci"/


\\ outs \\" outs "\’ outs ’

System.out.println("\\'comp\'sci\'/");

OUTPUT\'comp'sci'/


Escape Sequencesfrequently used combinations

Name Use

\t tabs over five spaces

\n moves to front of next line

\b deletes previous character

\r moves to front of current line

\\ nets one backslash \

\" nets one double quote "

\’ nets one single quote ’


// single-line comments/* */ block comments

//this line prints stuff on the screenSystem.out.println("stuff");


// single-line comments/* */ block comments

/* this line prints stuff on the screen*/System.out.println("stuff");


System.out.printf("%s","compsci\n");

OUTPUTcompsci


Syntax errors occur when you type something in wrong, causing the code to not compile.

//missing semicolon - ; expectedSystem.out.println("stuff")

//case problem – should be Systemsystem.out.println("stuff")


Runtime errors occur when something goes wrong while the program is running.

//an out of bounds exception is thrownString s = "runtime_error";System.out.println( s.charAt(15) );

© a+ computer science - . public class compsci { } all java programs start with a class

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